"Charly" is based on Daniel Keyes's short story turned into a novel "Flowers for Algernon." The story was about Charly Gordon, a man who, in the parlance of the time, was mentally retarded. As part of a scientific experiment he is given a drug that turns him into a genius. The Algernon of the title is a lab rat who was the first guinea pig for this treatment. However, the treatment proves to be only temporary. Both versions of Keyes's story were done as diary entries, which provided a graphic indication of how Charly is changing.
The 1968 movie version, of course, opens up the story and gets away from the first-person perspective that made "Flowers for Algernon" so compelling. To add insult to injury, there is now a romance between Charly with a character named Alice Kinian (Claire Bloom). Of course, this changes the whole dynamic of the film, at the cost of the poignancy of Charly's relationship with Algernon. As the title character Cliff Robertson won the Oscar and clearly the problem is not with his performance but rather with Stirling Silliphant's screenplay. Still, to be fair, any film adaptation of the fragile original story was going to lose what made it so great.
Consequently, this is one of those films that you will enjoy more if you have not read "Flowers for Algernon." Of course, if you have not read either the short story or the novel, you should. At least this was an intelligence "science fiction" film for its day, certainly a more human story than other films of that era, such as "2001: A Space Odyssey."